This invention relates to voltage measurement devices, and more particularly to voltmeters capable of measuring AC and DC voltages.
On many occasions, service personnel or other users of voltage measuring equipment need to know whether a particular line carries AC line voltage, DC voltage, or DC voltage with an AC ripple, as well as the magnitudes of these voltages. Further, such information is frequently needed on an urgent basis, or under circumstances wherein it is inconvenient for the user to manipulate various dial settings on a voltmeter.
The prior art has accordingly developed a number of AC/DC voltage reading voltmeters. For example, U.s. Pat. No. 3,536,998 to Nordholm discloses a measuring system for AC and DC voltages. The system is complex, however, requiring the use of a pair of polarity sensing circuits and a reversing switch assembly which provides an output to a meter circuit through a voltage-range control. The disclosed system assumes that incoming voltage is either AC or DC, and filters out all AC components when a DC measurement is being performed. The presence of AC voltage is detected by noticing that both polarity-sensing circuits have been activated.
U.S. Pat. No. 3,614,610 to Legatti discloses a voltage tester providing neon light indicators of the range of voltage operation as well as the polarity and frequency of operation. Voltages below the minimal operating range are indicated by flashing of yet a further indicator. The patented device, however, provides no display of the actual voltage magnitude. Moreover, no provision is made for a situation wherein a voltage includes both AC and DC components.
U.S. Pat. No. 4,092,591 to Lozowski provides separate LED indicators of particular AC or DC voltage ranges and otherwise suffers from problems similar to those in U.S. Pat. No. 3,614,610.
A simplified AC/DC meter is described in U.S. Pat. No. 3,345,562 to Rockwell. In this disclosure, however, no indication is provided of whether the displayed voltage is AC or DC. There is thus neither an identification of the particular function being performed by the meter nor an identification of the type of voltage being measured thereby.